1. Field of the Invention
The present invention relates to a lens barrel for driving a focusing optical system by using only one set of a cam groove and a cam pin engaged with the cam groove, the cam groove and the cam pin jointly regulating the zooming and focusing operations of the focusing optical system.
2. Description of the Related Art
In the lens barrel of this type, a magnification of an image seen through the lens barrel has been adjusted by moving at least a part of the optical system back and forth in the direction of an optical axis so as to change a focal length.
In such a lens barrel, when the range of a magnification to be adjusted is wider, the moving distance of the optical system is longer. This implies that compared with a lens barrel having the range of a focal length from 35 mm to 70 mm, the extended/contracted level of an external size is larger for a lens barrel having the range of a focal length from 28 mm to 80 mm. Also, compared with the lens barrel having the range of a focal length from 35 mm to 70 mm, the moving amount of the optical system is larger for a lens barrel having the range of a focal length from 70 mm to 105 mm, that is, the lens barrel having a larger focal length and a higher magnification, even though the changing amounts of focal lengths are the same between these lens barrels.
Another part of the foregoing optical system performs a focusing operation according to a focusing operation performed from the outside, in addition to the above-noted focal length adjusting operation (hereinafter, this optical system is referred to as "a focusing optical system"). A driving mechanism for driving the focusing optical system is generally composed of a lens moving frame for supporting the optical system, a cam pin installed in the lens moving frame, a cam groove for defining the position of the cam pin in the direction of an optical axis and a focus ring (driving member) having a guiding groove formed in parallel with the optical axis with the tip of the cam pin engaged with the guiding groove. In this driving mechanism, the focus ring is rotated manually or automatically, whereby power of the focus ring in the rotation direction is converted into power in the direction of the optical axis by the cam pin and the cam groove, and then this power is transmitted to the lens moving frame through the cam pin. As a result, the lens moving frame is moved in the direction of the optical axis and focusing is performed.
Thus far, for the lens barrel of this type, there has been produced mainly one having the small moving amount of a lens group and a low magnification, which enables the external size of the barrel to be formed compact. In recent years, on the other hand, there has been an increasing demand for a lens barrel, which is capable of changing a focal length in a wide range from a low magnification to a high magnification.
However, there was a problem inherent in the foregoing conventional lens barrel. The problem is as follows.
If the range of a focal length to be adjusted is made wider, the moving amount of the optical system becomes larger following the focal length adjustment as described above and the focusing optical system moves to the front of the lens barrel by a large amount. Thus, it is also necessary to make longer the guiding groove formed in the focus ring toward the front of the lens barrel according to the movement of the focusing optical system. As a result, the length of the focus ring in the direction of the optical axis is increased and the lens barrel cannot be formed to be compact.